The digital geological map of the southern sector of the Mesa Central province is presented, covering an area of ~41 000 km2 in central Mexico. This first interactive map is a compilation of the geological maps available in the literature and the unpublished maps done by our work team. The map focuses on the Cenozoic stratigraphy, supported by a geochronological database of 261 isotopic ages derived from K-Ar, U-Pb, Ar-Ar, and fission tracks methods. The interactive map displays the lithostratigraphic and chronostratigraphic units and the major and second-order Cenozoic faults. Map construction considered lithostratigraphy and geochronological correlation criteria and the main unconformities. Integrating these data in a single digital map allows a regional vision of the southern Mesa Central, making the map a valuable work tool to better understand the Cenozoic geologic evolution of central Mexico.
The understanding of each geological-structural aspect in the field is fundamental to be able to reconstruct the geological history of a region and to give a geological meaning to the data acquired in the outcrop. The description of a brittle extensional environment, which is dominated by normal fault systems, is based on: (I) image interpretation, which aims to find evidence suggestive of an extensional geological environment, such as the presence of scarp lines and fault scarps, horst, graben and/or half-graben, among others, that allow the identification of the footwall and hanging wall blocks; ii) definition of the sites of interest for testing; and iii) analysis of the outcrops, following a systematic procedure that consists of the observation and identification of the deformation markers, their three-dimensional schematic representation, and their subsequent interpretation, including the stereographic representation in the outcrop. This procedure implies the unification of the parameters of structural data acquisition in the field, mentioning the minimum fields necessary for the registration of the data in tables. Additionally, the integration of geological and structural observations of the outcrop allows to understand the nature of the geological units, the deformation related to the extensional environment and the regional tectonic context of the study area.
The description of the fabric elements represented by the linear and planar structures present at different scales is a key component of fieldwork. A scheme is proposed for the systematic registration of planes and lineations, coded as S (planar surfaces), F (folds), and L (lineations), among others, that allows for the orderly storage of the measurements taken. This scheme includes information related to the kinematics, the kinematic indicators, and the certainty or reliability ascribed to the assigned movement. In the fieldwork, the graphic representation of the structural measures in modified projection nets includes concentric circles for each dip. Direct drawing of the outcrop data is undertaken, dispensing with the use of tracing or transparent paper. The stereograms resulting from the graphic representation in the modified Wulff stereographic projection net, and the modified Schmidt equal-area net, can be complemented by rose diagrams for visualization of the spatial ordering. During field campaigns in the outcrops, it is essential to visualize the spatial orientation of the data in the diagrams to determine the main structural trends, the vergence, the kinematic nature of faults and shear zones, paleo-stress tensors, and to differentiate structural domains, among others. This information supports the reconstruction of geological and tectonic history and the establishment of relationships between the different geological processes.
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